1. Field of the Invention
The disclosures herein generally relate to a CPT resonance generation method, a CPT resonance detection method, a CPT resonance generation apparatus, an atomic oscillator and a magnetic sensor.
2. Description of the Related Art
An atomic clock is an extremely accurate clock. Technologies for reducing the size of the atomic clock are being studied. The atomic clock is an oscillator based on the transition energy of an electron included in an atom of alkali metal or the like. In particular, the electron transition energy of an alkali metal atom is found to be quite accurate if the process is not influenced by any disturbance. Accordingly, stability of the frequency, which is an improvement of several orders of magnitude compared with a crystal oscillator, can be obtained.
Because a conventional atomic oscillator requires a structure of a microwave oscillator, the atomic oscillator of the related art has a large size and requires a large amount of electric power. However, by using an atomic resonance called a CPT (Coherent Population Trapping), the oscillator becomes unnecessary, and it becomes possible to manufacture an atomic oscillator having quite a small size. A prototype of the CPT type atomic oscillator was manufactured in 2007, and products of the CPT type atomic oscillator have been sold by Symmetricom, Inc., U.S.A. since 2011.
The CPT type atomic oscillator includes, as shown in FIG. 1, a light source 910 such as a laser light-emitting element, an alkali metal cell 940 in which alkali metals are encapsulated, and a light detector 950 which receives laser light that passes through the alkali metal cell 940. The laser light from the light source 910 is modulated by sideband wavelengths appearing on both sides of a carrier wave which are specific wavelengths, and excites electrons in alkali metal atoms with two simultaneous transitions of the electrons.
The transition energy of the above transition is constant. When the sideband wavelength of the laser light coincides with a wavelength corresponding to the transition energy, a transparency phenomenon occurs, in which absorption of light by the alkali metal is reduced. Then, while a wavelength of the carrier wave is adjusted so that the absorption of light by the atomic metal is reduced, a signal detected by the light detector 950 is fed back to a modulator 960, and a modulation frequency of the laser light from the light source 910 such as a laser element is adjusted by the modulator 960. The laser light is emitted from the light source 910 and is transmitted onto the alkali metal cell 940 via a collimator lens 920 and a quarter wavelength plate 930, and enters the light detector 950.
Although, the CPT atomic oscillator is smaller and consumes less power in comparison to oscillators of the related art, oscillators of the related art exhibit better frequency stability, and further improvement of frequency stability in CPT oscillators is desired. As a powerful method for improving the frequency stability, a method for pulsing a laser light has been studied.
For the CPT type atomic oscillator, there are mainly two methods of pulsing laser light. A first method of pulsing laser light is to use an external apparatus. In the first method, a laser light-emitting element is caused to continuously emit light and the wavelength of the laser light-emitting element is caused to coincide with the absorption line of the atom, and thereby the frequency is stabilized. The laser light is pulsed by causing an external device such as an Acousto-Optic Modulator (AOM) or a liquid-crystal polarizer to intervene. In the first method, it is easy to stabilize the laser wavelength, but there is a problem of increasing a volume, cost and power consumption.
A second method of pulsing laser light is to use a direct modulation. The second method does not require a specific device other than the laser light-emitting element, and enables reducing the size, cost and power consumption of the entire apparatus, compared with the case of using an external device. As an example of the second method, a technique of superposing a microwave with a direct electric current input to the laser light-emitting element to perform modulation is proposed (for example, see US Patent Application Publication No. 2013/0056458).